Metabolism of urea in sheep

1. The entry rates of urea into the urea pool of the body fluids have been measured in sheep given rations varying in crude protein percentage from 3.5 to 27.3. 2. Results obtained with a single injection and with continuous infusions of [ 14 C]urea were essentially the same. 3. The difference between the entry rate and the rate of excretion of urea in the urine was taken to indicate the quantity of urea degraded in the alimentary tract. 4. Plasma concentrations and urea entry rates were significantly and linearly related. 5. The relationship between excretion rate and plasma urea concentration was best described by a cubic equation. 6. Degradation of urea in sheep was found to be extensive in all the animals studied; as the protein intake increased, the quantity of urea degraded also increased but the percentage of urea entering the body pool that was degraded was decreased. Animals given a ration containing 3.5% crude protein degraded 76–92% of the urea entering the body pool. 7. A rectilinear relationship was found between pool size and plasma urea concentration. The urea space in animals given low-protein rations was significantly less than in animals on high-protein rations. 8. The effects of starvation for 2, 4 and 6 days on urea metabolism in sheep were investi-gated. In a11 the sheep starved for 2 days there was a significant increase in urea pool size, but the entry rate was markedly depressed indicating a retention of urea in the body pool on starvation. 9. A significant amount of nitrogen was found to go through the system: rumen ammonia → portal blood ammonia→blood urea→rumen ammonia. 10.Urea excretion rate, urea clearance by the kidney, urine flow rate and the ratio of the concentration of urea in urine to that in plasma (urea U:P ratio) were also examined. 11. There were significant correlations between urine flow rate and urea excretion and between plasma urea concentration and urine flow rate.